710 THE EYE. 



The lachrymal sac, the upper dilated portion of the passage, is situated at 

 the side of the nose, near the inner canthus of the eye, and lies embedded 

 in a deep groove in the lachrymal and upper maxillary bones. It is of an oval 

 form ; the upper end closed and rounded, and the lower end gradually 

 narrowiug somewhat into the nasal duct. On the outer side, and a little in 

 front, it receives the lachrymal canals ; and here it is covered by the tendo 

 palpebrarum, and by some of the inner fibres of the orbicular muscle of the 

 lids; while on its inner or posterior. surface the tensor tarsi muscle is placed. 

 The sac is composed of fibrous and elastic tissues, adhering closely to the 

 bones above mentioned, and strengthened by fibrous processes sent from the 

 tendo palpebrarum, which crosses a little above its middle. The inner sur- 

 face is lined by a reddish mucous membrane, which is continuous through 

 the canaliculi with the conjunctiva, and through the nasal duct with the 

 mucous membrane of the nose. 



The nasal duct (ductus ad nasum), about six or seven lines in length, 

 grooving the upper maxillary bone, descends to the fore part of the lower 

 meatus of the nose, the osseous canal being completed by the ungual and 

 lower turbiuated bones. A tube of fibrous membrane, continuous with the 

 lachrymal sac, adheres to the parietes of this canal, and is lined by mucous 

 membrane, which, at the opening into the nose, is often arranged in the 

 form of an imperfect valve. The nasal duct is rather narrower in the middle 

 than at either end ; its direction is not quite vertical, but inclined slightly 

 outwards and backwards. 



The mucous membrane in the canaliculi possesses a laminar epithelium, 

 but in the nasal sac and duct a ciliated epithelium as in the nose. 



Various valves have been described in connection with the lachrymal sac and canals. 

 One, the valve of Hasner, is formed by the mucous membrane of the nose overhanging 

 the inferior orifice of the nasal duct, and has had imputed to it the function of pre- 

 venting entrance of foreign matters in violent expiratory movements; but the 

 disposition of the mucous membrane at this orifice appears to be subject to some 

 variation. Another fold, the valve of Huschke, placed at the deep orifice of the canali- 

 culi, is supposed by some to prevent the return of the tears from the sac into those 

 tubes, but by others, it is declared to be inconstant, and insufficient, even when found, 

 to close the orifice. A third fold, the valve of Foltz, is described as forming a projection 

 inwards on one side of the vertical part of each canaliculus, near the punctual 

 lachrymale, and as being sufficient to close the tube when it is flattened by the 

 pressure of the fibres of the orbicularis and tensor tarsi muscles as in winking. The 

 experiments of Foltz on rabbits go to prove that the punctum lachrymale having been 

 turned backwards towards the eye in winking, and the canaliculus being compressed 

 by the muscles, as soon as the pressure is removed the canaliculus resumes its open 

 form, and so sucks in tears which by the next compression in winking are forced 

 onwards into the lachrymal sac ; and also, that when the muscles are paralysed, the 

 canaliculi cease to carry away the tears. See review of Foltz 's paper in Dublin 

 Quarterly Journal, Feby. 1863; also, Hyrtl, Topogr. Anatomic. 



THE GLOBE OF THE EYE. 



The globe or ball of the eye is a composite structure of an irregularly 

 spheroidal form, placed in the fore part of the orbital cavity, and receiving 

 the thick stem of the optic nerve behind. The recti and obliqui muscles 

 closely surround the greater part of the eyeball, and are capable of changing 

 its position within certain limits : the lids, with the plica semilunaris and 

 caruncle, are in contact with its covering of conjunctiva in front ; and 

 behind it is supported by a quantity of loose fat and connective tissue. 



